This invention relates to tachometers in general and more particularly to a digital tachometer utilizing a row of light emitting diodes to provide a display which appears in the form of a bar graph.
Tachometers are required for many purposes where rotational speed is to be measured. Various types of tachometers are known, working both on mechanical and electrical principles. For example, a simple electric tachometer can be formed by driving an electrical generator which then develops a voltage proportional to speed which can then be displayed on a meter. Similarly, mechanical tachometers are known in which a cable is connected to a rotating part and an indication of speed transmitted mechanically to a display.
One place where tachometers find application is in automobiles where it is desired to measure the engine rpm. Tachometers are also used in other vehicles having internal combustion or turbine engines such as in aircraft. It has been typical in tachometers used with internal combustion engines having an ignition system, i.e., gasoline engines, to utilize the firing pulses to the engine to generate a tachometer signal. Thus, for example, circuits have been developed in which pulses from the distributor are sensed, possibly shaped, rectified, and the rectified voltage used to drive the meter. In such a circuit, because of the rectifier, an average DC voltage corresponding to rpm is developed. Other circuits have also been developed which, in other ways, average the output pulses of a distributor in order to obtain an analog DC voltage capable of driving a meter movement. It will be recognized, that tachometers of this nature have a limited accuracy which is directly related to the components involved and possible aging or change in value of the resistors, capacitors, etc., used in the device.
In an attempt to overcome this problem there have been attempts to develop digital tachometers. However, in each case, the digital display was by means of segmented digital displays driven by a decoded binary count. One of the major disadvantages of such a device is the poor readability of the display due to rapidly changing numbers. Other problems have been found in sensitivity to interference and cost. In one system which was tested and found to be unsatisfactory, a phase locked loop was employed to phase lock the divided output of a voltage controlled oscillator driving a counter to the incoming pulses from the distributor. The counter output was then latched at a predetermined time and the latch outputs used to drive a display.
It is thus evident that there is a need for an improved tachometer, useful particularly in automotive applications, which does not suffer from the deficiencies of prior art tachometers.